A DC-DC converter, which, through switching control of a semi-conductor device, converts an input DC voltage into a desired stable DC voltage, has such advantages as high efficiency and easy reduction in dimension and weight, and therefore is used as an essential constituent in power supplies for various electronic devices, in controlling electrical motors based on inverter technology, and in circuits to light various discharge lamps.
FIG. 16 is a circuit diagram of a typical step-down DC-DC converter 100. The DC-DC converter 100 includes a field-effect transistor Q1 as a main switching element, a flywheel diode D3, a choke coil L1, an output capacitor C5, and a control circuit 102, wherein a voltage Vi is a DC power source, R1 is a load, and C1 is a junction capacitance formed between the drain and source terminals of the field-effect transistor Q1.
The DC power source Vi has its positive terminal connected to the drain terminal of field-effect transistor Q1 and has its negative terminal grounded. The field-effect transistor Q1 is connected via its source terminal to the cathode terminal of the flywheel diode D3 and also to one terminal of the choke coil L1 which has its other terminal connected to one terminal of the output capacitor C5. The other terminal of the output capacitor C5 and the anode terminal of the flywheel diode D3 are grounded. The control circuit 102 is connected via its sensing terminal to the other terminal (positioned toward the load R1) of the choke coil L1, and via its output terminal to the gate terminal of the field-effect transistor Q1.
The DC-DC converter 100 operates as follow. Under a steady state condition with the field-effect transistor Q1 set turned-off, when the field-effect transistor Q1 is turned on, a current flows from the DC power source Vi to the choke coil L1 via the field-effect transistor Q1, and a voltage at the other terminal (positioned toward the load R1) of the choke coil L1 is smoothed by the output capacitor C5 and then applied to the load R1. While the field-effect transistor Q1 stays turned-on, energy is stored up in the choke coil L1 according to the current. Then, when the field-effect transistor Q1 is turned off, electromotive force is generated across the both terminals of the choke coil L1, and the current maintained by the electromotive force commutates to flow through the flywheel diode D3, whereby the energy stored up during the turn-on period of the field-effect transistor Q1 is supplied to the load R1.
With repletion of the operation described above, a voltage according to the duty ratio (on-time/on-time+off-time) of the field-effect transistor Q1 is outputted across the both terminals of the load R1. In order to keep the output voltage constant irrespective of variations of the input voltage (Vi) and the load R1, the control circuit 102 performs pulse width modulation (PWM) control, in which the duty ratio of the field-effect transistor Q1 is modulated according to a detected level of the output voltage.
In the DC-DC converter 100 described above, due to the junction capacitance C1 formed between the drain and source terminals of the field-effect transistor Q1 and also due to wiring-related parasitic inductances, a transitional period, at which a non-zero voltage across drain and source terminals and a non-zero drain current are concurrently present, arises at the moment when the field-effect transistor Q1 turns on or turns off, and a switching loss is thereby caused. Since the switching loss becomes larger with increase of a frequency for performing on-off control, a serious problem is involved when reduction of the dimension and weight of an apparatus is sought to be achieved by increasing the on-off control frequency so as to reduce the inductance of a choke coil and the capacitance of an output capacitor. Further, there is another problem that when the field-effect transistor Q1 turns off thereby reverse-biasing the flywheel diode D3, a large recovery current is caused to flow from the cathode to the anode at the reverse recovery time resulting in causing a heavy loss.
Under the circumstances described above, what is called a “soft switching technique” is conventionally applied which utilizes resonance thereby reducing losses attributable to the switching loss and the recovery current. For example, Japanese Patent Application Laid-Open No. 2003-189602 discloses a DC-DC converter in which a resonant circuit uses the junction capacitance of a switching element and a rectifying element in order to deal with an extensive range of an input and output voltage variation.
In the DC-DC converter disclosed in the aforementioned Japanese Patent Application, however, when the duty ratio is set small, a resonant coil cannot build up a sufficient energy thus resulting in a decreased voltage of a resonant capacitor. If this condition goes on, the voltage polarity of the resonant capacitor is reversed, and the resonant coil cannot be reset. Also, in the DC-DC converter, a rectification circuit, which is constituted by a rectifying diode (flywheel diode) and additional components for resonance, is structured such that only the resonant coil is located between a first switching element and a choke coil while the other components are connected in parallel to the resonant coil, and consequently if the rectification circuit is made into one module, then the circuit is to come with at least three terminals, which hampers an easy modification from a conventional circuit which incorporates only a flywheel diode that is a two-terminal element.